There has heretofore been known a technique where with the overlaid sheet-like members clamped and pressured between a backup surface of a stage and a pressing surface of a horn (pressing body), ultrasonic vibration is induced in a horn which applies the ultrasonic vibration to the sheet-like members and thus, the overlaid sheet-like members are welded together. In this case, the overlaid sheet-like members clamped and pressured between the pressing surface and the backup surface are welded together in one stroke by application of the ultrasonic vibration. However, the following problem is encountered when the sheet-like members are welded together over a wide area.
To weld together the overlaid sheet-like members over the wide area in one stroke, it is required to provide a large pressing body including a wide pressing surface such that the sheet-like members can be collectively pressured over the whole area of a welding range by pressing the sheet-like members against the backup surface in a pressing direction. In this case, however, the pressing surface of the ultrasonically vibrating pressing body exhibits a great difference in the magnitude of amplitude between spaced positions on the pressing surface as well as an increased phase difference therebetween. It is therefore difficult for the pressing surface to uniformly apply the ultrasonic vibration of the same amplitude to the sheet-like members over the whole area of a pressing range. Further, the provision of the large pressing body having the wide pressing surface also dictates the need to increase a driving force for driving the pressing body in the pressing direction in order that the wide pressing surface achieves a predetermined magnitude of pressure. Accordingly, support means for supporting the pressing body also needs to have a large structure, which leads to another problem that the apparatus per se is increased in size.
Accordingly, there has been proposed a method where the sheet-like members, as pressed against the backup surface in the pressing direction by the pressing surface smaller in the area than the welding range, are moved in a direction substantially orthogonal to the pressing direction whereby the sheet-like members are pressed against the backup surface over the whole area of the welding range (see, for example, Patent Document 1). According to an apparatus disclosed in the patent document 1, the overlaid sheet-like members moved in the direction substantially orthogonal to the pressing direction are welded together over the whole area of the welding range by being pressured by the pressing surface smaller in the area than the welding range. The driving force required for driving the pressing body in the pressing direction to apply a predetermined pressure via the small pressing surface is smaller than the driving force required for driving the large conventional pressing body. Since the compact pressing body having the small pressing surface is applicable to the apparatus, the overlaid sheet-like members can be welded together by the apparatus having a compact structure.